Numerical modeling of field ejection assisted by ultra-short laser pulses: Pushing the atom probe tomography to its highest capacities – Ultra-Sonde

The project is focused at the improvement of the laser assisted APT and in the widening of its capacities in terms of the resolution, size of the analyzed region, materials to be analyzed, and precision. The main scientific objective is to establish the relationship between (i) illumination characteristics, (ii) material properties, (iii) electron or electron-hole pair excitation mechanisms and carrier dynamics (iv) subsequent material ejection. To achieve these goals, a better understanding of both ultra-fast femtosecond interactions and field extraction processes is required. In particular, this understanding should lelp to provide a possibility to explain the time of flight spectra based on the confrontation of the experimental findings and numerical models; a possibility to study electron excitation, ionization and recombination in dielectric and semiconductor materials looking at the ions evaporation time, a possibility to reduce the absorption and thermal damage optimizing the laser parameters according to the numerical calculations of absorption distribution and tip temperature evolution and a possibility to increase the evaporation rate changing laser parameters (wavelength, polarization, pulse width) according to the field extraction theory. The main challenge of this project is in the development of a protocol to analyze all materials and new nanoelectronics devices with atomic resolution in space. All diagnostics need theoretical support to extract the mechanisms of the atomic ejection and to enhance the flux.
The objectives of the project require addressing the following scientific questions: (i) Excitation and ionization mechanisms occurring during the laser pulse (~100 fs) in the presence of a strong external field; (ii) Electron energy relaxation, electron-ion/matrix interactions and heating (femto- to picosecond time scale, ~1 ps); (iii) Subsequent material ejection and field extraction (up to nanoseconds). These processes occur at all time scales, from several femtoseconds.to nanoseconds Therefore, corresponding numerical models will be developed to provide a multi-scale simulation of the involved physical processes and, as a result to elucidate the basic mechanisms of material extraction. In particular, such techniques as detailed ab initio calculations of the density of states and the properties of the electronic sub-system, kinetic modeling of laser excitation and electronic transport, two-temperature thermo-dynamical calculations and molecular dynamics simulations of material ejection will be performed. As a result of the modeling, a better interpretation of the experimental results will be provided.